Internet security experts say that the computer worm known as Conficker, which has the ability to silently penetrate vulnerabilities within the Microsoft operating system, is beginning to rear its ugly head.

They say that the software is installing new and malicious programs on some of the computers it has already invaded with the aim of using those PCs to send out criminal spam and scrounge around on unsecured computers for valuable personal data, Reuters reported Friday.

Conficker, also called Downadup and Kido, works like this: Once the worm wiggles into a PC, it then has the ability to install software and enable the computer to receive additional viruses from the program’s creators. It can also link an individual PC to other infected machines and create an army of computers under its control, called a botnet, which can be strung together for launching cyberattacks.

Millions of PCs already invaded

Experts say that the Conficker worm has already dug into millions of PCs but only been activated in a small percent of them. It was feared that the makers of the software program would trigger a massive attack on April 1. While that didn’t happen, the US Computer Emergency Readiness Team (US-CERT) said earlier this month that it has detected a new variant of the worm that “updates earlier infections via its peer-to-peer network against unpatched systems.”